The concept of maintaining a cryogenic environment within a predetermined temperature range through the use of a heating or cooling element is known in the art. In U.S. Pat. No. 3,667,246, a cryogenic load is maintained at a predetermined temperature range by regulating the temperature of a coolant. In U.S. Pat. No. 4,306,425, a biological object is maintained at a predetermined temperature range by regulating the temperature of a metal receptacle. In U.S. Pat. No. 4,712,607, biological material is maintained at a predetermined temperature range by regulating the temperature of a space for receiving such materials. In U.S. Pat. No. 4,848,093, the temperature in a cryogenic test chamber is maintained at a predetermined temperature range by regulating the temperature of a fluid supplied to the test chamber by a capillary tube. In U.S. Pat. No. 5,101,638, a magnetic field generator is maintained at a predetermined temperature range by regulating the temperature of a thermal shield surrounding the generator.
As disclosed by the prior art mentioned above, objects are typically maintained within a predetermined temperature range by using a heating or cooling element to regulate the temperature of the cryogenic environment or materials surrounding the object. While this approach is sufficient for maintaining the object within a relatively wide temperature range, there are certain applications that cannot tolerate such a wide temperature range, because these certain applications depend on high performance circuits that are very temperature sensitive.
An example of such a high performance circuit is a high temperature superconductor circuit. These high temperature superconductor circuits are very temperature sensitive, although they do exhibit desirable properties when cooled to cryogenic temperatures. The term "high temperature" is used as a relative term, and generally refers to temperatures above 30 Kelvin (K), and more particularly, refers to temperatures above the boiling point of liquid nitrogen, which is approximately 77 K. To put Kelvins into a proper frame of reference, 77 K is approximately -320.degree. Fahrenheit (F.). Thus, even these so called "high temperature" superconductor circuits operate at extremely cold temperatures that require highly specialized equipment and control techniques for optimum performance.
Cryogenic operating temperatures are typically achieved by using cryogenic coolants stored in dewars to cool the environment surrounding the object. For high temperature superconductor circuits, the use of liquid nitrogen as a cryogenic coolant provides an acceptable operating temperature of approximately 77 K, or -320.degree. F.
One highly advantageous benefit of high temperature superconductor circuits is their low resistance. This property results in extremely selective and precise frequency responses in resonant circuits. A result of this beneficial feature is that very narrow frequency bandpass, or bandstop, filters can be formed from high temperature superconductor materials. In application, bandpass filters are desirable because they provide more channels in a narrow frequency spectrum, and bandstop filters are desirable because they reject interfering signals in a narrow frequency spectrum. High temperature superconductor technology is used in various types of circuits from narrow band filters to systems such as down converters.
Along with the benefits of high temperature superconductor circuits, there are also drawbacks. For example, minor variations, fluctuations, or deviations in the operating temperature have a direct impact on the performance of high temperature superconductor circuits.
The reason that high temperature superconductor circuits are highly sensitive to temperature fluctuations is due in part to the temperature dependence of the critical current density, which defines the maximum current density that can be carried before the superconductor material becomes resistive. Another factor is the temperature dependence of the magnetic penetration depth.
The frequency response of high temperature superconductor circuits are sensitive to temperature fluctuations far below their transition temperature. Furthermore, high temperature superconductor circuits are especially sensitive to fluctuations in their operating temperature when they are operating near their transition temperature, which is the temperature at which a high temperature superconductor circuit switches between a state of normal resistivity and a state of superconductivity.
Moreover, minor fluctuations in the operating temperature can shift the operating frequencies of certain high temperature superconductor circuits, such as filters and resonators, by several megahertz (MHz), adversely affecting the performance of such high temperature superconductor circuits.
Thus, it is desirable to operate circuits implementing high temperature superconductor materials within a predetermined temperature range, wherein the range of tolerance is plus or minus 0.1 K, a range that is currently very difficult to maintain using prior art cryogenic temperature control systems that control the temperature of the cryogenic environment or materials surrounding the circuit. With many of the prior art cryogenic temperature control systems, the relatively wide temperature range that is precisely maintainable is intolerably high for the proper operation of high temperature superconductor circuits in certain high performance applications.
Therefore, a better solution is needed for providing a system and method for maintaining the object within a predetermined temperature range in a cryogenic environment, wherein the range of temperature fluctuations is preferably less than .+-.0.1 K, thereby allowing certain high performance temperature sensitive circuits to perform within operational specifications.